When one wears a denture for a long time, the denture becomes unfitted to an oral membrane due to bone resorption and/or deformation of the surface of the membrane. For this reason, means for restoring an optimum fit of the denture by relining the surface of the unfitted denture base with a denture base relining material is clinically employed.
Illustrative examples of a material used in the denture base include an acrylic resin, a polycarbonate resin, and a polysulfone resin. Of these, the acrylic resin is the most commonly used. Meanwhile, as a material used as the relining material, a mixture of monomers and polymers of (meth)acrylic resins is predominantly used. To bond the relining material to the denture base effectively, a method of applying an organic solvent such as methylene chloride which contains halogen atoms to the surface of the denture base is generally employed.
However, when the denture base and the relining material are made of different materials, sufficient bonding strength may not be obtained through the application of the organic solvent containing halogen atoms alone. For this reason, as adhesives for general purpose applications, adhesives comprising organic solvents containing halogen atoms and a variety of polymers have been developed.
As such adhesives, an adhesive using a chlorine-based organic solvent and a polycarbonate resin (refer to JP-A 61-50906 and JP-A 61-134306), an adhesive using a combination of a chlorine-based organic solvent and an aromatic polyester polymer or a polyester carbonate (refer to JP-A-61-136562), an adhesive using a combination of methylene chloride, a polystyrene resin and a polymethyl methacrylate (refer to JP-A 58-72509), and an adhesion assistant using a combination of methylene chloride, a polycarbonate resin, a polymethyl methacrylate and a multifunctional methacrylate (refer to JP-A 10-231228) and other adhesives are known. In these adhesives, methylene chloride is the most widely used as the organic solvent containing halogen atoms since it has a low boiling point, evaporates fast and easy to handle.
However, in recent years, more stringent control of the organic solvent containing halogen atoms has been increasingly demanded in view of environmental loads and harmful effects. For example, the Ministry of Health and Welfare has provided “Guideline for Residual Solvents in Drugs” to all prefectural and city governments in 1998. In the guideline, residual solvents in drugs are classified into classes 1 to 3 based on their toxicity. According to the classification, methylene chloride used in the above adhesives is classified into the class 2 since it is a solvent which does not causes an unacceptable level of toxicity but shows a certain level of toxicity, and it is defined as a solvent whose residual level in drugs should be regulated so as to protect patients from possible harmful effects. Further, other chlorine-based organic solvents usable as adhesives for denture base relining materials are also classified into the class 1 or 2. Hence, an adhesive for a denture base relining material which uses no chlorine-based organic solvent is desired.
Further, as an adhesive using no chlorine-based organic solvent, an adhesive comprising an organic solvent having a boiling point of 30 to 250° C. and a thermoplastic polyhydroxy ether is disclosed as an adhesive for a polysulfone-based resin and a dental acrylic resin in JP-A 62-149609. The adhesive takes long time to dry after application and has lower bonding strength than an adhesive using a chlorine-based organic solvent.
Further, the following can be said of all polymer-containing adhesives. That is, bonding durability when such adhesives are used is not necessarily satisfactory. Further, there arises a problem that when the surface of a denture base coated with the treating agent is partially left uncovered with a relining material and the uncovered portion is exposed after completion of bonding operation, the uncovered portion (hereinafter referred to as “surface coated with an excess of treating material or adhesive”) may be whitened, thereby degrading esthetics of the denture base.
Meanwhile, as dental materials such as a denture base relining material, dental cement, a composite resin and a resin for crown, compositions comprising a radical polymerizable monomer which can be polymerized in the presence of a radical, e.g., (meth)acrylate, an organic peroxide and a tertiary amine compound are widely used due to their characteristic that they are polymerized quickly at temperatures ranging from room temperature to a temperature in an oral cavity so as to form cured products.
For example, a direct method denture base relining material is a material for repairing a base of a denture which becomes unfitted to the palate of a patient as a result of long-term use so as to restore the denture to a reusable condition. To use the material, a paste which is polymerized and cured at temperatures ranging from room temperature to a temperature in an oral cavity is applied on a denture base, the denture is directly inserted into the oral cavity of a patient and fitted to the surface of an oral membrane, and the paste is polymerized and cured while kept in the oral cavity so as to restore the denture base. As the relining material, the above compositions which are cured around room temperature are widely used. Heretofore, as such a relining material, a powder-liquid two-component material comprising a powder component which is a mixture of a powdery synthetic resin such as a polymethyl methacrylate (hereinafter abbreviated as “PMMA”), a polyethyl methacrylate (hereinafter abbreviated as “PEMA”) and a copolymer of methyl methacrylate and ethyl methacrylate (hereinafter abbreviated as “PMMA-PEMA”) and an organic peroxide and a liquid component which is a mixture of a radical polymerizable monomer such as methyl methacrylate and a tertiary amine compound has been used.
In the case of such a two-component material, when the two components were mixed together, radicals are produced easily at temperatures ranging from room temperature to a temperature in an oral cavity, and polymerization and curing start after a lapse of given time from the mixing. Further, upon contact with the powder component, the above liquid component not only dissolves a portion of the powder component in a short time but also permeates the synthetic resin powder so as to swell it. Thus, the viscosity of the mixture can be adjusted properly, and a clinical operation can be facilitated advantageously.
However, the above direct method denture base relining material has a problem that the material undergoes inhibition of surfacial curing caused by oxygen in the air and/or foreign matter in an oral cavity at the time of curing, whereby an insufficiently cured portion is liable to be formed. If such an insufficiently cured portion (hereinafter referred to as “surface unpolymerized layer”) exists, it causes such problems that a relined denture is liable to discolor and that foreign matter in an oral cavity is liable to stick to the denture and renders it unsanitary.
Such a problem of formation of the surface unpolymerized layer is a problem common to dental curing compositions containing radical polymerizable monomers, organic peroxides and tertiary amine compounds and has been desired to be improved.
As a method for preventing formation of the surface unpolymerized layer of the curable composition and reducing a surface unpolymerized amount (amount of unpolymerized monomer present on the surface), a method of applying an air barrier agent and a method of using an accelerator are known. The method of applying an air barrier agent is a method of covering the surface of a radical polymerizable monomer with an oxygen-shielding film formed by application of the air barrier agent during curing of the radical polymerizable monomer so as to prevent inhibition of polymerization caused by oxygen in the air. As the air barrier agent, an aqueous solution of a polyvinyl alcohol (refer to JP-A 58-201628), a composition prepared by adding an organic peroxide to a polyvinyl alcohol aqueous solution (refer to JP-A 59-36604), a composition comprising a low-molecular-weight polyethylene glycol and a high-molecular-weight polyethylene glycol (refer to JP-A 59-134705), and a composition comprising an aqueous solution of a water-soluble oxygen-shielding polymer, a water-soluble reducing agent and a surfactant (refer to JP-A 9-241304) are known. Meanwhile, the method of using an accelerator is a method in which a curable composition such as a denture base relining material is cured in an oral cavity and then a denture is immersed in a hot aqueous solution having an accelerator dissolved therein so as to cause the curable composition to undergo secondary curing. As commercial accelerators, a product which generates carbon dioxide upon dissolution in water and removes oxygen dissolved in the water and a product characterized in that a water-soluble polymerization initiator is dissolved in hot water have been actually used.